In DSL (Digital Subscriber Line) technology, such as VDSL2, the use of the spectrum available for the service is governed by a band plan, which is usually part of the standard. The plan specifies, for each end of the loop, which bands (frequency intervals) are intended to be used for transmission (pass bands) and which may not (stop bands). A PSD (Power Spectral Density) mask specifies the permitted transmit power for the bands.
Normally, some bands, upstream bands, are reserved for transmission by the CPE (Customer Premises Equipment), whereas other bands, downstream bands, are reserved for transmission by the DSLAM (Digital Subscriber Line Access Multiplexer) at the central office. Thus, a band which is a passband in the band plan for one end of the loop will be a stop band in the band plan for the other end of the loop.
There may be bands which are not intended for transmission at all (e.g. they are stop bands in the band plan for the central office end as well as being stop bands also in the band plan for the customer end). This may be to prevent interference with other systems.
Different band plans may be used depending on the setting, i.e. the circumstances and the location on the loop where transmission takes place. E.g. the PSD masks are (naturally) different for the customer and central office ends of the loop, but may also be different for different applications, such as fiber-fed cabinets near the customer end as opposed to copper connection all the way between customer and central office. Band plans and PSD masks may also differ between different markets or operators depending on regulatory requirements designed to limit interference to/from other equipment.
In general, it is not possible to design a transmitter to be absolutely quiet (i.e. to transmit no power at all) in some frequency bands while transmitting at normal power in others. In particular, when transmitting in a pass band at normal transmit power there will always be side lobes and intermodulation products in the adjacent stop bands, i.e. a leakage of power into the bands not intended for transmission. For this reason, the PSD masks allow a certain small transmit power also in the stop bands.
Single Ended Line Testing, SELT, is a method for testing a transmission line (loop) which requires actions at one end of the line only. Typically a signal of some kind is sent to the line and a resulting signal, such as an echo is received. From the relationship between the sent and the received signal, information about the line and its properties may be deduced.
In TDR (Time Domain Reflectometry) SELT, a brief pulse is sent to the line. The resulting echo as a function of time is recorded. Defects in the line may be seen as peaks in the echo curve, and the position in time on the curve gives information about where on the line the defect is located.
FDR (Frequency Domain Reflectometry) SELT is a method wherein a more or less stationary signal covering many frequencies is applied to the line for a certain time. The resulting reflected signal as a function of frequency, is recorded (i.e. amplitude and phase as a function of frequency).
The received signal as a function of frequency is then divided by the sent signal as a function of frequency, to get an echo frequency response. Various adjustments and other manipulations can be made in order to represent different line properties. By use of the inverse Fourier transform, the impulse response of the line may be generated mathematically.
When making a measurement, there is often a desire for greater accuracy, to be able to better resolve the location and other characteristics of faults and other line artifacts.
EP 1 111 808 A1 describes a TDR (Time Domain Reflectometry) method wherein specially designed edge regions of TDR pulses makes it possible to add the corresponding time domain echoes to produce a wideband impulse response measurement. This allegedly solves the problem of how to detect more than just the first defect on a line.
ITU-T standards G.993.2 and G.992.5 describe the VDSL2 and ADSL2+ standards respectively.